The effect of self-control on individual behavior has long been a subject of debate. Previous investigations have led to three main models of self-control, with contradictory predictions. By perceiving self-control as a knowledge, skill, and perishable resource, those models make assertions of a positive, neutral, and negative impact, respectively, of an initial act of self-control on subsequent self-control ability. Using biometric data enabled us to settle this controversy by developing a unified model of self-control that reconciles the diverging results in the literature. By monitoring actual compliance with an initial self-control task, our model overcomes the main limitation of assuming full compliance with the self-control treatment and restricting the analysis to linear relationships. The results suggest a dual impact of an initial act of self-control on subsequent self-control ability. Specifically, while an initial moderate self-control act enhances subsequent self-control ability, exerting self-control beyond a certain threshold causes fatigue and reduces subsequent self-control ability.